Superheterodyne frequency changing circuits



Jan. 16, 1934. w. VAN B. ROBERTS 1,943,458

SUPERHETERODYNE FREQUENCY CHANGING CIRCUITS Filed Feb. 19, 1930 A BY Patented Jan. 16, 1934 u STATES vSIlPERI-ITERODYNE FREQUENCY CHANGING CIRCUITS Walter van B. Roberts, Princeton, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application'February 19, 190. Serial No. 429,522

5 claims. (ol. 25o-2o) This invention relates to a superheterodyne receiving circuit in which the intermediate frequency is derived in a new and efficient manner.

The primary object of this invention is to provide means for obtaining an intermediate frequency in a superheterodyne receiving circuit by combining the local oscillator frequency and the incoming signal modulated radio frequency without the possibility of interference from signals of other than the desired wave length.

Other objects and novel features which I believe to be characteristic of my invention are set -forth in what follows and with partilcularity in Vthe appended claims. l

My invention, however, as to its organization .and method of operation will best be understood by vreference to the following description taken in connection with the accompanying drawingin which;

Figure 1 shows a typicalfrequency changing .arrangement for superheterodyne reception;A

Figure 2 shows Figure 1 doubled, however, .with the plate circuits in opposition; and, Y

Figure 3 shows a preferred embodiment of my invention-in which the intermediate frequency is obtained from the desired signal and the local oscillatorfree from intermediate frequency currents produced from undesired signals.

In Figure .1, an antenna l is shown coupled to a tuned circuit .2 Vwhich is tuned to the signal. 'As will-be noted tuned circuit 2 forms a portion 'of the input circuit of space discharge device 4. A local oscillator 3 is provided for impressing locally generated oscillationsV in the input cir cuit of tube 4. AThe output of tube 4, a portion ofwhich is constituted by the tuned circuit 5,.is tunedrto an intermediate frequency and as is -usually the custom the tuned output circuit is coupled to an intermediate frequency amplifier of any desired e, form through. a transformer 6. `It .is a disadvantage of thissimple arrangement that even if the local oscillations or heterodyne lis'notappliedy intermediate frequency currents may be produced in the output circuit of the tube v4 bya simultaneous receptionof two incoming yfrequencies differing from each other by the frequency to which the intermediate frequency ampliiier is tuned, or bythe simultaneous reception of two frequencies whosel sum is equal to the intermediate frequency, or by the reception cfa/single frequencyk one of whose harmonics is equal to the intermediate frequency, or by the `reception cfa signalwhich is equal to the, interu mediate frequency. As is evident these effects h are likely to becauses of serious imperfections in the operation of the simple superheterodyne circuit illustrated in Figure 1.

In addition Yto the disadvantages enumerated above there is the possibility of intermediatefrequency currents being produced by combinations of various frequencies applied to the gridof tube 4. ,Howeven as to these latter `mentioned difliculties it is well known that in the majority of cases they are notl apt to be seri-ous. In'the present state of the art it is customary tominimize the rst mentioned effects by employing a signal selecting` system capable of such selectivity that only the desired frequency reachesthe grid of the frequency -changing tube and in addition to this so choosing the intermediate fre'- quency thatA it willnot be equal to any harmonic of any of the signal frequencies; to be received;`

In what follows the term symmetrically coupled or equivalent expressions whenvapplied to the inputs of the two space discharge vdevices is to be construed as meaning that the energy impressed upon said inputs through said coupling affects theA grids of the devices in phase consonance. Y 1

In Figure 2 I have shown a system somewhat similar to adoubled showing of Figure 1 except that the .output circuits of the tubes `rPand-8 are connected in opposition to each other.y In saidgure, 1 represents theV antenna inwhich high frequency signal energy is received,f9 corresponds to the tuned circuit 2 of Figure 1 except that in the case of Figure 2 the voltages impressed upon tuned circuit 9 are inV turn inrpressed upon the grids of the vtwo space dis,- charge devices 7 and 8 in-like sense, and tuned circuit 10 of Figure 2 corresponds to the Voutput circuit 5 of Figure 1.7 It will be noted that coil L; of circuit 10 is arranged so as to be onehalf in the output circuit of device 'land one half in the output of device 8. Coupled to the l'coil L1 by means of a coil Liarrangedso as to be coupled to both halves of L1 is anintermediate frequency amplifier circuit. It will be noted that vin Figure 2 since upon the receipt of a signal in antenna 1 the same charge is placedupon each of the grids of tubes '7 and 8 and since the output circuits of said tubes-.are connected in opposition any and all output currents result.- ing from the application Mofvvoltage-to thegrids are cancelled out in the differential transformers L1, L2 provided that ythe connection to L1 from the B battery 1l and the couplings between 'L2 and the two, halves of L; are so .adjusted that the `effect of voltage onone grid. in producing Vvoltage in isexactly neutralized by thev effect ofthe voltage on the other grid. The simplest case where the above cancellations of output currents occurs is where the two tubes 7 and 8 used have identical characteristics and the so-called plate current source is connected to the midpoint of L1, and L2 is coupled to the two halves so formed of Li. It will be evident that with the arrangement shown in Figure 2 freedom from intermediate frequency currents in L2 produced by any combination of input frequencies or harmonies thereof is achieved. However, in such an arrangement there are no means provided for producing intermediate frequency currents in response to the desired signal.

To obtain intermediate frequencies from the desired signal while retaining freedom from intermediate frequency currents produced in the undesired fashions enumerated in connection With the description of Figure l, I have devised the scheme shown in Figure 3 in which 1 represents the antenna for receiving high frequency energy, '7 and 8 are the two space discharge devices connected as described in Figure 2, 10 is the tuned output circuit to which by means of' L1, L2 is coupled an intermediate frequency amplifier and 11 is the usual B battery. However, Figure 3 differs from Figure 2 in that a local oscillator 3 is coupled to the input circuit of" tube 7. In Figure 3 when the source of local oscillations or so-called heterodyne is connected as shown intermediate frequency current will be produced in the tube 'l by the beats between the heterodyne and the signal. It is evident that the same results will be obtained when the heterodyne is applied to both grids in the same phase while the signal is applied to the input ofthe tube '7 which result can be achieved by simply interchanging the positions of the heterodyne and the signal inputs of Figure 3. However, the better practice is the arrangement as shown in Figure 3 because if the two energy sources are intel-changed, in the absence of the heterodyne the odd harmonics of the signals including the fundamental are not cancelled out bythe differential transformer. The circuit of Figure 3 is furthermore preferred for the reason that the rotor of the tuning condenser may be kept at an unvarying potential.

It will' be noted that in my invention as above described an 'untuned input circuit may be employed in place of the tuned input circuit in which case Various stations may be selected by simplyY varyingV the heterodyne frequency. The only serious interference which may result from not tuning the signal input is the possibility of intermediate frequency being produced byV an interfering frequency as much higher than the heterodyne as the heterodyne is higher than the desired signal. Of course, this is assuming that the heterodyne is to be operated at a frequency above that of the signal. There is also the possibility of intermediate frequency current being produced by an interference whose frequency plus the frequency of the heterodyne is equal to the intermediate frequency. However, in such a case by suitable choice of the intermediate frequency all such possible interfering frequencies may be made to lie outside the range of frequencies which are to be received so that by the use of a xed band pass filter or its equivalent located between the antenna 1 of Figure 3 and the input 9 of the frequency changing system all of these interfering frequencies may be suppressed without the recourse of any element that requires reacljusting when changing from one desired signal to another. The band pass lter, of course, must be designed so as to pass all the desired frequencies. It will be evident that by this arrangement a superheterodyne receiver may be built which can be tuned by the variation of only one element, namely, the heterodyne tuning condenser and which is nevertheless free from the more serious sources of interference which occur when the same result is attempted using the simple frequency changing arrangement shown in Figure 1.

It is to be understood, that the invention is not limited to the particular application shown.

I claim:

l. In a superheterodyne receiver a frequency changing circuit comprising a pair of electronic tubes, push-pull output circuits therefor, input circuits for said tubes including a common portion, a source of high frequency signal energy, a source of heterodyning frequency energy, means for impressing energy from one of said sources across the common portion of the input circuits of both tubes whereby energy from said source affects both tubes in phase consonance,

means for impressing energy from the other of said sources across a portion of the input circuit of only one of said tubes and an intermediate frequency amplier differentially coupled to the push-pull output circuit of said tubes.

2. In a superheterodyne receiver, a frequency changing system comprising a pair of space discharge devices arranged in push-pull, a circuit tuned to the frequency of desired incoming signal oscillations connected in the commonv portion of the input circuits of said devices, the output circuits thereof being tuned to a desiredl intermediate frequency, a circuit for the reception of high frequency signals coupled to saidl rst mentioned tuned circuit and a circuit containing a local source of high frequency energy coupled to the input circuit of one of said devices.

3. In radio receiving apparatus of the superheterodyne type a frequency changing system comprising a pair of space discharge devices, push-pull output circuits therefor, input circuits for said devices including a common portion, an energy absorption system, a circuit tuned to the frequency of desired incoming signal oscillations connected in the common portion of said input circuits whereby energy impressed thereon will affect both said space discharge devices equally and in the same sense as regards phase and amplitude, means for coupling said absorption .130

common portion, a circuit for the reception of signalling energy, a source of heterodyning frequency energy, means for coupling said reception circuit to the common portion of the input circuits of said space discharge devices, means for coupling said heterodyning circuit to the input circuit of one of said devices, an intermediate frequency ampliner and means for coupling said amplifier to the push-pull output circuits of said space discharge devices.

5. A frequency changer circuit for superhet- 'it' erodyne receivers and the like comprising a pair @f translating devices having input and output `circuits, a circuit for the reception of high frequency signals, a second circuit containing a local source of high frequency energy, means for symmetrically coupling one of said circuits to Said input circuits whereby energy transferred through said means affects both devices in phase consonance, means for coupling the other of said circuits to the input circuit of only one of said devices and a beat frequency amplifier circuit differentially connected to said output circuits.

WALTER vAN B. ROBERTS. 

